1. Field of the Invention
The present invention relates to a process for preparing oxide powders which are used as starting materials for producing high temperature superconducting ceramics and to a process for fabricating the superconducting oxide ceramics.
The present invention also relates to processes for producing c-axis oriented superconducting film and bulk materials.
2. Background of the Related Art
Recently, superconducting oxide ceramics which show superconductivity at or above the liquid nitrogen temperature have been found. For example, YBa.sub.2 Cu.sub.3 O.sub.7-x and LnBa.sub.2 Cu.sub.3 O.sub.7-x (Ln: a lanthanoid) have been reported to be superconductors having a superconducting transition temperature (T.sub.c) at about 90K. These high-T.sub.c superconducting oxides are favorable for wide commercial application, because the price of the liquid nitrogen used as the cooling medium is about 1/10 of that of liquid helium. These superconductors are fabricated from oxide powders as starting materials. Since the properties of the superconductors themselves are strongly influenced by the starting materials, not only is the fabrication process for the superconductors being widely investigated, but also the method for producing the starting oxide powders.
A solid phase reaction method and a coprecipitation method are known techniques for producing oxide powders for superconductors. The fabrication process of the solid phase reaction method is as follows; a mixture of oxides, such as CuO, Y.sub.2 O.sub.3 (or Ln.sub.2 O.sub.3) and BaCO.sub.3, in a certain ratio is sintered at a temperature of 950.degree.-1000.degree. C. to make YBa.sub.2 Cu.sub.3 O.sub.7-x or YLn.sub.2 Cu.sub.3 O.sub.7-x in a solid phase reaction at a high temperature, and then the sintered product is slowly cooled to room temperature and is ground into powder. The fabrication process of the coprecipitation method is as follows; oxalic acid is added to an aqueous solution of Cu(NO.sub.3).sub.2, Ba(NO.sub.3).sub.3 and Y(NO.sub.3).sub.3 or Ln(NO.sub.3).sub.3 to precipitate metallic ions as oxalates, and then the resultant precipitants are dried and calcinated at a temperature of 900.degree.-1000.degree. C. to decompose the oxalates into oxides, and the products are slowly cooled and ground into powder.
In the solid phase reaction method, a one hundred percent reaction of the components cannot be accomplished and, hence, unreacted oxides are often present in the product. On the other hand, while in the coprecipitation method more homogeneous products can be obtained than in the solid phase reaction method, the starting composition and the composition of the products are different, which is caused by the higher solubility of barium oxalate than the solubility of the other components.
From the point of view of crystalline phase change, in both of these techniques, a non-superconducting phase (tetragonal phase) appears upon sintering at 900.degree.-1000 .degree. C. during the first stage. Then, the tetragonal phase is transformed into a superconducting phase (orthorhombic phase) by absorption of oxygen during the subsequent slow cooling process in an oxygen-rich atmosphere. The reason why a sintering temperature higher than 900.degree. C. is necessary is that the large particle size of the starting powders made by these techniques results in low activity in the reaction.
Moreover, after grinding of the calcined products, the particle size of the powder is on the order of a few micrometers and the size distribution is not as sharp. Therefore, to make a dense product from the powder made by these techniques, re-sintering at a temperature higher than 1000.degree. C. is required. Since such a high temperature sintering often generates other crystal phases (not the YBa.sub.2 Cu.sub.3 O.sub.7-x or LnBa.sub.2 Cu.sub.3 O.sub.7-x phase), however, sometimes the T.sub.c of the sample was lower than the liquid nitrogen temperature.
Meanwhile, for forming a superconducting film, various techniques have been investigated including a method using a reaction in the gas phase, such as the plasma spray coating technique, a vacuum evaporation technique, a sputtering technique, and coating methods, such as the screen printing technique and the spin coating technique. These techniques include a heating process after the formation of the film on a substrate, and have some problems. In the vacuum evaporation and the sputtering techniques, the composition of the film is often different from the starting material composition. Furthermore, in the sputtering process, crystal structure and orientation of the resultant film are strongly influenced by the substrate temperature. The screen printing technique also has the problems that a dense film cannot be obtained easily because of the large particle size of the starting powder, and that the critical current density of the film made by this technique is lower than that of a film formed by sputtering. Another technique for producing super-conducting film has been proposed; organometallic compounds, such as yttrium stearate, barium naphthenate and copper naphthenate, are dissolved into a suitable solvent and are coated on a substrate and heated to decompose into an oxide film. This technique, however, has the problem that carbon remains in the film because of the strong reducing atmosphere in the sintering process.
When silicon or silica glass are used as substrates for superconducting oxide films, the superconducting phase cannot be obtained because of the reaction between the copper in the oxide film and the substrates during the sintering process. Accordingly, substrates having low reactivity during sintering, such as yttrium-stabilized zirconia (YSZ) or the like, are used as a substrate for film forming. Such a type of substrate, however, has the following problems: (1) since the substrate is an insulator, for a superconducting device involving circuit application, another substrate which is semiconducting is required; and (2) since it is difficult to make variously shaped substrates, such as fiber or tape, from ceramics, the substrate shape is limited to that of a plate.
For practical application of the superconducting material, a high critical current density and a high critical magnetic field should be secured. It is known that if the superconducting crystal is c-axis oriented and a current is flowed along a surface perpendicular to the c-axis, critical current density is improved. Therefore, it is desirable that the superconducting crystal have a particular orientation. Although, when the YBa.sub.2 Cu.sub.3 O.sub.x powder is pressed into a pellet shape, c-axis orientation has been observed, no method has been reported to realize c-axis orientation effectively.